Generally, a carbon nanotube has optical non-linearity 1000 times higher than that of other materials, such that the carbon nanotube is appropriate for use as a saturable absorber. In addition, in the carbon nanotube, a band-gap thereof is determined according to a diameter of the carbon nanotube and a rolling vector of a graphite plate due to a small size of a nano scale and a specific carbon bond. Since this band-gap is significantly small (0.4 eV or less) as compared with other semiconducting materials, the optical non-linearity may be significantly increased. In addition, since the nanotube may be produced so as to have various sizes while changing the diameter, the band-gap may be changed, such that the carbon nanotube has variability with respect to band-widths in which mode-lock may be performed. Further, generally, when a saturable absorber is interlocked with an existing optical device to be used, damage cannot help but occur at an external environment and a periphery junction part. However, the carbon nanotube may minimize this damage due to excellent mechanical strength as described above. In addition, the carbon nanotube may be easily produced, have significantly low producing cost, as compared with a semiconductor saturable absorber mirror (SESAM), and be easily combined with a fiber laser system in a film form, or the like.
The saturable absorber used in passive mode-locking system is a non-linear optical medium of which absorption is decreased when light intensity is increased. In the case in which the saturable absorber is inserted into a cavity, a pulse width may be shortened while pulse shuttles in the cavity, such that an ultra-fast light pulse may be generated.
As conditions of the saturable absorber for a mode-lock, the saturable absorber should have an absorption rate more than a gain constant of a semiconductor laser and a recovery time faster than a carrier relaxation time of the semiconductor laser. In the existing fiber laser system, the SESAM has been mainly used for the passive mode-lock. However, in the SESAM, a wavelength band in which the mode-lock may be performed is determined according to a thickness of a stacked semiconductor layer, but production of the semiconductor appropriate for the wavelength region of 1.3 to 1.5 um, which is a wavelength generally used in the fiber laser, requires a complicated process, such that producing cost may be high. In addition, it may be almost impossible to vary the wavelength band in which the mode lock may be performed, and it is difficult to combine the SESAM with the fiber laser system, such that the SESAM has many limitations.
In order to overcome this problem, the saturable absorber using a carbon nanotube has been mainly produced. A method of producing the saturable absorber using the carbon nanotube may be classified into two types, that is, a composite type method and a spray type method.
The composite type method is a method of co-dispersing a liquid polymer and a single-walled carbon nanotube (SWNT) and curing the dispersant to thereby produce the saturable absorber. In this method, the curing should be performed at a constant temperature for 1 week, such that it takes a long time to produce the saturable absorber. In addition, since the polymer and the SWNT are not uniformly mixed but sporadically mixed, incident laser may be irregularly reflected. Further, it may be difficult to adjust a thickness or optical absorbance of the saturable absorber at a desired degree.
The spray type method is a method of producing a saturable absorber by directly spraying a single walled carbon nanotube on a thin film polymer to dispose the single-walled carbon nanotube on the polymer. This method has a disadvantage in that it may be difficult to obtain a uniform surface due to characteristics of the spray type method, such that scattering of the laser may be generated, and it may be difficult to obtain the desired optical absorbance similarly to the composite type method.
As the related art, an ultrafast carbon nanotube saturable absorber for solid-state laser mode-locking has been disclosed in KR 10-2010-0043446. The related art relates to a carbon nanotube saturable absorber obtained by forming a carbon nanotube solution by mixing a single-walled carbon nanotube (SWCNT) produced from an electric discharge with diclobenzene (DCB), mixing the carbon nanotube solution with polymethyl methacrylate (PMMA) to produce a SWCNT/PMMA composite, and forming a thin film with the SWCNT/PMMA composite by a spin coating method on a substrate. In this case, it may be difficult to adjust the thickness or optical absorbance of the saturable absorber at a desired degree.